Cracking and Alkenes

Crude oil fractions are not produced in the proportions needed by consumers:

• There is high demand for short-chain hydrocarbons (petrol, gases)
• There is low demand for long-chain fractions (fuel oil, bitumen)
• Supply does not match demand

Cracking converts long-chain hydrocarbons into:

• Shorter alkanes (for fuel)
• Alkenes (for making plastics and other chemicals)

Catalytic Cracking

• Long-chain hydrocarbon vapour is passed over a hot catalyst (zeolite/aluminium oxide)
• Temperature: about 600–700°C
• Produces branched alkanes, cycloalkanes, and alkenes
• Used mainly for making motor fuels

Steam Cracking

• Long-chain hydrocarbons are mixed with steam
• Heated to very high temperatures: 800–900°C
• Produces lots of alkenes (important for making plastics)

Example Equation

$\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$

(Decane → octane + ethene)

$\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_7\text{H}_{16} + \text{C}_3\text{H}_6$

(Decane → heptane + propene)

Note: You must check that the number of C and H atoms balances on both sides.

Alkenes are a homologous series of unsaturated hydrocarbons. They contain at least one carbon-carbon double bond (C=C).

General Formula

$C_nH_{2n}$

Saturated vs Unsaturated

Bromine Water Test

• Add bromine water (orange) to the substance
• If the substance is an alkene: bromine water turns from orange to colourless
• If the substance is an alkane: bromine water stays orange (no reaction)

The C=C double bond reacts with bromine in an addition reaction:

$\text{C}_2\text{H}_4 + \text{Br}_2 \rightarrow \text{C}_2\text{H}_4\text{Br}_2$

(Ethene + bromine → dibromoethane)

The product is colourless, so the solution decolourises.

Because alkenes have a C=C double bond, they can undergo addition reactions — a small molecule adds across the double bond, opening it into a single bond.

Types of Addition Reactions

Hydrogenation (+ H₂): $\text{C}_2\text{H}_4 + \text{H}_2 \xrightarrow{\text{Ni catalyst}} \text{C}_2\text{H}_6$ Alkene + hydrogen → alkane (used to make margarine from vegetable oils)

Hydration (+ H₂O): $\text{C}_2\text{H}_4 + \text{H}_2\text{O} \xrightarrow{\text{acid catalyst}} \text{C}_2\text{H}_5\text{OH}$ Ethene + water → ethanol (used to produce ethanol industrially)

Halogenation (+ Br₂): $\text{C}_2\text{H}_4 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCHBr}$

Many alkene molecules can join together in a process called addition polymerisation to form very long chains called polymers (plastics).

How It Works

• Many small alkene molecules (monomers) join together
• The C=C double bond in each monomer opens up
• Monomers link to form a long chain with only single bonds
• The long molecule is the polymer

Example: Poly(ethene) from Ethene

$n\text{C}_2\text{H}_4 \rightarrow (-\text{CH}_2-\text{CH}_2-)_n$

Many ethene monomers → poly(ethene) (commonly called polyethylene)

Example: Poly(propene) from Propene

$n\text{C}_3\text{H}_6 \rightarrow (-\text{CH}_2-\text{CH(CH}_3\text{)}-)_n$

Common Polymers

Problems with Polymers

• Most polymers are not biodegradable — they don't break down naturally
• They fill up landfill sites
• Burning polymers can release toxic gases
• Recycling is difficult because different types must be separated

• In cracking equations, count C and H on both sides to check balance
• Remember: bromine water test — orange to colourless = alkene
• Addition polymerisation: C=C opens up, monomers link as single bonds
• Know the difference: saturated (only single bonds) vs unsaturated (contains C=C)

• Cracking breaks long-chain hydrocarbons into shorter alkanes + alkenes
• Alkenes ($C_nH_{2n}$) are unsaturated with a C=C double bond
• Bromine water test: decolourises with alkenes, no change with alkanes
• Addition reactions: small molecules add across the C=C bond
• Addition polymerisation: many alkene monomers → long polymer chain
• Environmental issues: polymers are non-biodegradable, fill landfill